Due to increased power generation by unsteady renewable sources like wind or solar existing gas turbine based power plants are increasingly used to balance power demand and to stabilize the grid. Thus improved operational flexibility is required. This implies that gas turbines are often operated at lower load than the base load design point, i.e. at lower combustor inlet and firing temperatures.
At the same time, emission limit values and overall emission permits are becoming more stringent, so that it is required to operate at lower emission values, keep low emissions also at part load operation and during transients, as these also count for cumulative emission limits.
State-of-the-art combustion systems are designed to cope with a certain variability in operating conditions, e.g. by adjusting the compressor inlet mass flow or controlling the fuel split among different burners, fuel stages or combustors. However, this is not sufficient to meet the new requirements.
To further reduce emissions and operational flexibility sequential combustion has been suggested in DE 10312971 A1. Depending on the operating conditions, in particular on the hot gas temperature of a first combustion chamber it can be necessary to cool the hot gases before they are admitted to a second burner (also called sequential burner). This cooling can be advantageous to allow fuel injection and premixing of the injected fuel with the hot flue gases of the first combustor in the second burner.
Conventional cooling methods either require heat exchanger structures which lead to high pressure drops in the main hog gas flow or suggest injection of a cooling mediurn from the side walls. For injection of a cooling medium from the side walls a high pressure drop is required which is detrimental to the efficiency of a gas turbine operated with such a combustor arrangement and a controlled cooling of the whole flow is difficult.
With reference to WO 2014/063835 A1 a sequential combustor arrangement comprising a first burner, a first combustion chamber, a mixer or admixing a dilution gas to the hot gases leaving the first combustion chamber during operation, a second burner, and a second combustion chamber arranged sequentially in a fluid flow connection. The mixer is adapted to guide combustion gases in a hot gas flow path extending between the first combustion chamber and the second burner comprising a duct having an inlet at an upstream end adapted for connection to the first combustion chamber and an outlet at a downstream end adapted for connection to the second burner. The mixer comprises a plurality of injection pipes pointing inwards from the side walls of the mixer for admixing the dilution gas to cool the hot flue gases leaving the first combustion chamber.
In addition WO 2014/063835 A1 describes a method for operating a gas turbine with at least a compressor, a combustor arrangement comprising a first burner, a first combustion chamber, a mixer for admixing a dilution gas to the hot gases leaving the first combustion chamber during operation, a second burner and a second combustion chamber arranged sequentially in a fluid flow connection. The mixer is adapted to guide combustion gases in a hot gas flow path extending between the first combustion chamber and the second burner comprising a duct having an inlet at an upstream end adapted for connection to the first combustion chamber and an outlet at a downstream end adapted for connection to the second burner. The mixer comprises a plurality of injection pipes pointing inwards from the side walls of the duct for admixing the dilution gas to cool the hot flue gases leaving the first combustion chamber, and a turbine. The dilution gas is admixed into different regions of the cross section of the mixer or the dilution gas is injected through injection holes and/or second injection pipes and first injection pipes in order to introduce dilution gas into different regions of the cross section of the mixer.
Furthermore, attenuation of combustion dynamics by making use of Helmholtz dampers are widely used by all the companies operating in the gas turbine sector and several invention disclosures have been filed. The emerged state of the art focus on damper applications to can-annular-combustors. Adopting a Helmholtz damper around a can-combustor can be seen in the following documents:
US 2005/0166596 A1 describes a through-flow resonator, which when placed at or near the points having the highest acoustical pressure amplitudes within the flow path of a combustor system, effectively dampens instabilities from acoustic energy while avoiding an increase in mass air flow bypassing the system when a portion of the resonator is integral to a section of the system having a thickness in excess of about 0.10 inch. The acoustic damping performance of a resonator may be expressed in terms of acoustic conductance, which is defined as the in-phase component of volume velocity through the downstream set of holes, divided by the amplitude of pressure oscillation at the downstream face of the resonator. A high value of acoustic conductance indicates high damping performance. Consequently, the higher the acoustic conductance of a resonator, the fewer individual resonators required within a system, such as a gas turbine, to minimize potentially damaging combustion oscillations, or the greater the likelihood against the occurrence of such combustion oscillations.
US 2011/0220433 A1 provides the following solutions: A first aspect of the present object is a combustor including a cylindrical body that defines a combustion area therein, and an acoustic damper that includes an acoustic portion having an acoustic-damper resonance space communicating with the combustion area. The acoustic portion is provided along the cylindrical body so as to extend in a direction intersecting an axial direction of the cylindrical body. According to this aspect, because the acoustic portion having the acoustic-damper resonance space is provided along the cylindrical body so as to extend in the direction intersecting the axial direction of the cylindrical body, or the circumferential direction, the acoustic portion is disposed widely in the circumferential direction, without concentrating in a particular area of the cylindrical body in the circumferential direction. As a result, the acoustic portion is prevented from protruding toward the outer circumference of the cylindrical body, and the space needed outside the combustor can be reduced. Thus, because the casing can be made small, the housing constituting the casing can be made small. Because this enables, for example, the gas turbine to be adequately transported on the ground, it is possible to reduce the manufacturing costs, including the transportation costs. Furthermore, if the protrusion of the acoustic portion toward the outer circumference of the cylindrical body is reduced, the combustor can be easily extracted together with the acoustic damper. Thus, it is possible to improve the ease of maintenance of the combustor. The above-described aspect may further include an acoustic liner formed by a porous plate that constitutes the cylindrical body and has a plurality of through-holes penetrating in a thickness direction and a cover member that is provided around and at a certain distance from the porous plate so as to cover the porous plate, the acoustic liner having an acoustic-liner resonance space. By doing so, it is possible to attenuate oscillations in a frequency region that can be attenuated by the acoustic liner and oscillations in a frequency region that can be attenuated by the acoustic damper. Accordingly, it is possible to attenuate combustion oscillations in a wide frequency region. In the above configuration, it is preferable that at least part of the acoustic portion be provided on the outer circumferential side of the acoustic liner.
Accordingly, feeding directly from compressor plenum does not leave enough air for sequential liner cooling. Used air from sequential liner cooling increases risks of hot gas ingestion, overheating in regions with low pressure drop and coupling between hot gas and mixer air plenum. Another solution could be a major pressure drop increase over dilution air, but this releases detrimental effects on engine performance.